Typically it means that given some situation or quality, an unexpected and negative thing exists as well. Look at these examples on alc:

# お金持ちのくせにけちだ
have deep pockets and short arms (i.e. "he has a lot of money but he's stingy")
# 営業のくせに情報が遅いじゃない。
For a salesman, you're uninformed, aren't you?
# 子供のくせにたばこを吸う
smoke when someone is a mere child
# 新人のくせに態度がでかい
act big for a newcomer
# 新人のくせに態度が大きい
act big for a newcomer
# 男のくせに泣く
cry like a girl

What I am getting from the above examples is that if one is X, one should not be Y, for being X means that one is better than Y, thus the negative implication by using this expression. But one thing confuses me:

コムスメのくせに大した怪力だ。

This example is pulled from the anime Naruto during a fight between a man and a young woman. The man says the above to the the woman after the woman displays her "superhuman strength", as she easily deflects his large-scale, levitating-steel attacks against her.

According to my naive knowledge of Japanese, he seems to be saying:

コムスメだから、強くないハズだ。

He would be only expressing surprise, then? There does not seem to be any apparent negativity in that meaning (aside from calling her コムスメ.) Did I interpret it incorrectly?